The basic feature of all coherent processing techniques is the computation of a frequency domain statistic which represents an estimate of magnitude-squared coherence .vertline..gamma.(.omega.).vertline..sup.2 between two or more time series. If a common narrowband component with center frequency .omega..sub.c is present in each time series, with the remaining energy in each series uncorrelated, then the true coherence will be nonzero only for frequencies around .omega..sub.c. Values of the coherence statistic at discrete frequencies .omega..sub.j (j=0, 1, . . . , K-1) are computed over the frequency range of interest and then values above threshold are taken to determine detections. Each time series is derived from the output of a distinct sensor or array of sensors and the various sensor outputs provide the input signals to a multi-sensor or multi-array correlator.
It has been shown that the detection performance for the stationary Gauss-Gauss and fixed processing parameters depends solely on the signal-to-noise ratio (SNR) of the narrowband component in each time series. By optimum matching of the processing bandwidth to the signal bandwidth, these SNRs are maximized. However, for reasons including both data nonstationarities and computational burdens, the processing bandwidth used in coherent multi-sensor correlation processing is generally larger than the anticipated signal bandwidth. This raises the interesting possibility of adaptively prefiltering the inputs to the multi-sensor coherence processor to enhance the SNRs of all narrowband components contained therein, thus recouping some of the loss due to bandwidth mismatches. It will be shown herein that such an approach is feasible. Specifically, it will be shown that a special type adaptive linear predictive (ALP) filter, namely an adaptive line enhancer (ALE), substantially increases the useful signal output. The adaptive line enhancer is described by J. R. Zeidler and D. M. Chabries, "An Analysis of the LMS Adaptive Filter Used as a Spectral Line Enhancer", report Number NUC TP556, which was published in February 1975 by the Naval Undersea Center, San Diego, Calif., 92152.